The present invention relates to ophthalmic lenses having more than a single focal length. The methods and structures provided are applicable to proximal and spectacle lenses and other lenses for correcting human vision.
The majority of vision-correcting lenses are designed to correct sight solely for distance viewing—with a single focal length. When a person views near objects, the eye accommodates the divergent, rather than parallel, light arriving from the near object. The act of accommodation by the human eye results in a physical change in shape of the crystalline lens within the eye, the flexure of this lens causes the incoming divergent light emitted by near objects to re-converge and focus on the retina. Accommodation increases the convergence of light waves by causing the lens surfaces to be more steeply curved, which in turn adds focal power to the optical system of the eye. The closer an object is viewed, the greater the accommodative demand placed on the eye. As the human crystalline lens ages, it slowly loses its gel-like flexibility. Although the process goes unnoticed for the better part of four decades, the lens body expands in girth and hardens, losing the ability to change shape with a resulting loss in accommodative ability. This condition is known as presbyopia. Typically, corrective lens wearers begin to notice presbyopia near the end of the fourth decade and then begin to require more than one lens in order to see clearly and efficiently at all distances. The convergent focal power requirement of this multiple lens system then typically increases gradually over the next fifteen years.
Early versions of multiple corrective spectacle lens systems for the human eye simply added an additional spectacle lens below the distance lens and designated the two-lens system a bifocal. The additional focal power afforded by this arrangement was known as the add-power for near vision. Eventually a third lens was placed between these two lenses to improve vision at intermediate distances, and the system became a trifocal. Because of recent innovations in the field of ophthalmic lens design, spectacles are now available in multifocals that are made in a continuous array of focal powers. These spectacles are made to accommodate the eye for distances from infinity to the reading plane, and are known as progressive-addition lenses. Although multifocal spectacle lenses have been largely successful in satisfying the needs of spectacle wearers, multifocal lenses that are positioned on or in the eye (proximal lenses): contact lenses, intra-ocular lenses and the alternative surgically imparted corneal lenses have been much less successful. Many recently emerging presbyopes are life-long wearers of contact lenses that correct only the distance vision. As presbyopia develops, most of these patients are forced to wear reading glasses over their contact lenses or to wear a distance lens in one eye and a near lens in the opposite eye (mono-vision). These two modes are very inconvenient, and sometimes hazardous to the wearer. Wearers of the so-called mono-vision modality must necessarily forfeit the natural binocular function that is needed to judge depth. Another growing population in need of multifocal correction is the pseudo-aphakic post-cataract surgery patient whose natural lenses have been replaced with implanted polymeric lenses. These patients must wear spectacles for reading after successful surgery, but many of them could benefit if the implanted lenses were efficiently shaped multifocals. Such a multifocal implant must be capable of replacing the variable focusing ability of their youthful natural lenses. Yet another large and growing group of lens wearers are the recent recipients of corneal surgery who find themselves forced to wear reading glasses after a very expensive surgical procedure that corrects only the distance vision. If corneal surgery could introduce a multifocal into the corneal stroma of proper shape and focal power distribution, it would alleviate the necessity of wearing reading spectacles in post-operative presbyopes.
Previous attempts to provide multifocal power to the human eye using contact lenses or other proximal lenses (those on or in the eye) have had limited success. Mimicking the simple bifocal spectacle, the device as described in U.S. Pat. No. 4,693,572 to Tsuetaki, et. al. is an example of segmented alternating bifocal contact lenses. This type of lenses must be caused to translate on the cornea of the wearer by pressure from the sensitive lower lid margin when the wearer gazes downward. Despite the discomfort of the lip pressure, this design has experienced some niche success. That success comes in part from the wide field of vision afforded the wearer as the pupillary aperture is exposed to a very large component of either the lower (near) lens or the (upper) distance lens in the two positions of gaze.
More recent designs, as found in U.S. Pat. No. 5,436,678 to Carroll depend upon the phenomenon of simultaneous focus to obtain addition power along with distant vision correction. Using this method, multiple foci: far, near and intermediate are presented within the pupillary zone at the same time. These devices depend upon discrimination by the cortical vision system to select the best focus available for the distance that is being viewed. Though this approach has had considerable success, most designs can correct only moderate amounts of presbyopia and are usually most successful when the surface shape treatment is applied to the corneal side of rigid gas-permeable contact lenses. In these designs, extreme curvatures are applied to the base curve, often creating metabolic problems for the cornea. The devices of the Carroll patent are modified in U.S. Pat. No. 5,835,187 to Martin to include a plurality of spherical zones on the front surface while maintaining conic curves on the posterior surface so that multifocal addition power is obtained from both surfaces. Unfortunately, the spherical zones are pieced together and are not a continuous array or radii with continuous first and second derivatives, with the result that diffraction will play a roll in degrading the optical performance of these lenses.
Various shapes have been applied to lens surfaces to improve simultaneous focus lenses. Aspheric multifocal lenses have been designed using multiple zones of conicoid surfaces. In concentric designs such as contact lenses or intra-ocular lenses, adjacent surfaces of revolution of differing shapes are smoothed mathematically to develop addition power that increases radially on the lens surfaces. Conicoids evolved as usable shapes for ophthalmic lenses primarily because of their variable shape and innate ease of manipulation. Consequently, aspheric contact lenses evolved from these conic functions. Though highly effective in generating variable foci, lenses designed around conic shapes do not always provide acceptable optics for the eye and can be somewhat unwieldy when used with interconnecting lens surfaces. Examples of more successful bifocal (not multifocal) lenses are discussed in U.S. Pat. Nos. 5,448,312 and 5,929,969 to Roffman. The Roffman bifocal is generated by alternating rings of two radii, one for distance power and another for near power in such a way as to maintain excellent near power for typical pupil sizes and ambient light conditions. Distance vision suffers from diffractive effects caused apparently by the centermost rings that surround the distance power zone, and by the loss of light and optical clarity created by the grooves between radii. An improvement on this design would seek to use an aspheric central and intermediate zone with alternating rings in only the outer add-power zone.
In most multifocus lenses adjacent power zones have boundaries which induce diffraction and other optical aberrations which degrade visual acuity. Various smoothing and transition methods have been developed to reduce this problem. U.S. Pat. No. 5,815,236 to Vayntraub discloses use of a logarithmic function in defining smoother transitions between lens zonal curves. U.S. Pat. No. 4,640,595 to Volk discloses using a variable shape (e-value) in smoothing conicoid surfaces. U.S. Pat. No. 5,452,031 to Ducharme discloses use of piece-wise polynomials or splining techniques to smooth zonal curve transitions. Unfortunately, the optical areas taken up by these transitions are at best wasted for vision correcting benefit and typically still introduce unfocused regions that reduce overall visual clarity. Optical discontinuities and ineffective transitions are particularly problematic within or adjacent to lens regions used for distance vision where they are more perceptible to the user than within near vision regions. Clear distance vision requires clear optics and the preponderant myopic population will not tolerate distance blur. Good pupil economics is also essential for the success of any lens placed on or within the eye itself. Given the limited size of the pupillary aperture, an array of lenses introduced to the eye-optical system must be applied with great precision and without wasted or unused optical area.
Other devices have been studied for the improvement of distance vision. The ability of the eye to see distance more clearly with a relatively fixed small aperture is well known. Consequently, methods of correcting distance vision have been proposed that use pinholes devices or similar small aperture designs. U.S. Pat. Nos. 3,794,414 to Wesley and 5,192,317 to Kalb provide examples of this approach. Although benefits can be gained for correcting presbyopia, most of these designs suffer from defects caused by diffraction at the edge of a dark ring or masked area, a phenomenon that detracts from any possible improvement obtained from these small aperture designs. In addition, the perimeter masking that is used to create these devices precludes multifocal functioning that is desired for correcting presbyopia.
Lenses made according to the above mentioned patents and methods reflect the optical limitations resulting from a large number of requirements for clear human vision at all distances and under a wide range of light conditions. These conditions can be more nearly met if the special attributes and abilities of the eye are fully utilized and applied economically within the limited pupillary zone. Uniquely, the human vision system is comprised of a rather simple optical device that often needs optical correction because of its diminutive size and organic changes. This rudimentary device is yoked to a complex cortical vision system that can control and suppress blurred areas advantageously if presented with carefully designed optics. What is needed is a method of forming an ophthalmic lens providing multiple focal lengths without loss of optical efficiency or acuity from ineffective transition regions, particularly adjacent distance vision regions. Preferably, such a lens also effectively creates a small aperture to improve viewing at any distance without the problems inherent to typical small aperture devices.